B 2 0648 + 27 : a radio galaxy in a major merger . ⋆

We present WSRT observations of the neutral hydrogen in the nearby radio galaxy B2 0648+27. In emission, we detect a very large amount of H I (MHI = 1.1 · 10 10 M⊙) that is distributed in a very extended disk, or ring-like structure, of about 160 kpc in size. We also detect H I absorption against the central radio continuum component. The detection of the H I, its structure and kinematics, give us key information for building a possible evolutionary scenario. The characteristics of the detected H I are explained as the result of a major merger event that is likely to have occurred < ∼ 10 9 yr ago. Interestinly, we find that, when observed in radio continuum at higher resolution, this galaxy has a double lobed, steep spectrum structure of about 1 kpc in size. Thus, despite its low radio power, B2 0648+27 bears striking similarities with Compact Symmetric Objects, i.e. objects believed to represent the early phase of radio galaxies (< ∼ few thousand yrs old). B2 0648+27 is one of the few nearby radio galaxies where extended neutral hydrogen has been detected so far. This, and other recent results, appear however to indicate that nearby radio galaxies are more often gas rich than commonly assumed. The phenomena described are likely to be much more common at high redshift and galaxies like B2 0648+27 may provide valuable information on the evolution of high redshift radio sources. 1. Mergers and the origin of radio galaxies Galaxy mergers and interactions are often invoked as trigger of the nuclear activity in radio galaxies. The large number of powerful radio galaxies showing peculiar optical morphology (tails, bridges, shells and dust-lanes; Smith & Heckman 1989; Heckman et al. 1986), together with the detection of large amounts of CO in a number of them (Mazzarella et al. 1993, Evans et al. 1999a,b), can be taken as evidence for this. The increasing evidence that super-massive black holes reside in the centres of many elliptical galaxies (e.g. Kormendy & Richstone 1995) and the detection of potential fuel for AGN (ionized gas and dust) in both radio loud and quiescent galaxies (see e.g. Verdoes Kleijn et al. 2002a and references therein), suggest that radio activity may occur (perhaps as a short phase) in the life of many (or even all) elliptical galaxies. The nature of the triggering events remains, however, uncertain. It is not clear …